In the treatment of fiber suspensions, particularly comminuted cellulosic fibrous suspensions (such as paper pulp), it is desirable to be able to effect mixing of treatment fluids with the suspension at "medium" consistency, which is approximately the consistency from the digester. Typically, the consistency range will be about 6-15%. If mixing can effectively be accomplished at such consistency, there is no need for complicated apparatus and procedures for diluting, and then rethickening, the suspension.
Typical apparatus for effecting mixing of treatment fluids with suspensions--such as shown in Canadian Pat. No. 1,102,604--includes a rotor mounted for rotation with a rotatable shaft which extends along the centerline of a first diameter portion of a housing, through which the suspension passes. The rotor includes a plurality of generally axially extending, radially upstanding, ribs, and preferably the inner wall of the housing includes stationary ribs that are also axially extending, and are generally radially inwardly directed, and radially spaced from the rotor ribs. The rotor is rotated at an RPM sufficient to effect fluidization of the suspension, so that when the treatment fluid is introduced appropriate intermixing is achieved despite the fact that the suspension is at a consistency of about 6-15%.
While some conventional mixers utilizable with medium consistency pulp effectively accomplish their objectives, intermixing is sometimes accomplished only at a higher RPM than desired, and with consumption of more power than desired. It has been found, according to the present invention, that unnecessarily high RPMs and power consumption can occur as a result of a "shadow" being formed behind the rotor ribs and/or stationary ribs, as the rotor rotates within the suspension. Under some circumstances, a plug of suspension can actually result, and this plug can remain intact and actually "leak" through the entire mixing zone without being fluidized, and intermixed with the treatment chemical.
According to the present invention, a means is provided for eliminating the "shadow" formed behind the rotor and stationary ribs, so as to ensure efficient transmittal of the energy of rotation of the rotor to the areas behind the ribs (in the direction of rotation of the rotor) so that power consumption is minimized, and so that the rotor can rotate at a minimum RPM while still effectively accomplishing fluidization of the suspension, and intermixing of the suspension with treatment fluid. Elimination of the shadow is accomplished by forming the rotor ribs so that each has a first side wall that is generally radially extending, a generally flat top, and a second side wall, opposite the first side wall, that is generally at a tangent to the rotor surface. The first side wall of each rotor rib is closest to the second side wall of the circumferentially adjacent rotor rib, and vice versa. The rotor is rotated in a direction of rotation such that the second side wall of each rib is the "trailing" wall.
The desired results according to the present invention are also accomplished by forming the stationary ribs so that the second wall thereof is a wall that slopes from the generally flat top of the rib to the inner wall of the housing, occupying the volume where the "shadow" would normally form during rotation of the rotor with respect to the stationary ribs.
It is the primary object of the present invention to provide a mixing apparatus, and mixing method, that minimize power consumption and rotor RPM during the effective treatment of medium consistency fibrous suspension with treatment fluid. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.